1. Field of the Invention
The present invention relates to recording data in and reproducing data from an optical disc, and more particularly, to an apparatus which records data in and reproduces data from an optical disc having a wobbled track, and a method of controlling the apparatus.
2. Description of the Related Art
Generally, a recording density of an optical disc increases as the diameter of a laser beam decreases. To decrease the diameter of a laser beam, a short-wavelength laser source is used, and the numerical aperture (NA) of an objective lens of a pickup device is increased. As the diameter of a laser beam decreases, a focus error with respect to a recording layer increasingly influences the degradation of a signal quality. Accordingly, a majority of optical disc recording and/or reproducing apparatuses possess a device which compensates for a focus error.
A spherical aberration which occurs due to a change in the thickness of an optical disc is proportional to λ/(NA)4. For example, where an optical disc has λ=400 nm and NA=0.85, a thickness deviation is strictly limited within ±3.4 μm because a signal quality is greatly degraded where the thickness deviation of an optical disc goes beyond ±3.4 μm. Usually, optical discs include a plurality of recording layers to increase the recording capacity. Here, a distance between the recording layers is about several tens of micrometers. In the case of optical discs having a plurality of recording layers, a thickness deviation goes beyond an allowable range. Accordingly, an accurate recording or reproducing is not impossible. Therefore, a recording and/or reproducing apparatus for an optical disc having a plurality of recording/reproducing layers requires a device which compensates for a change in the thickness of the optical disc.
FIG. 1 shows a conceptional diagram of a method of compensating for a focus error in a conventional recording and/or reproducing apparatus. Referring to FIG. 1, a radio frequency (RF) signal is used to compensate for a focus error of a pickup device (not shown) provided in the conventional recording and/or reproducing apparatus. Beams which have been reflected from a recording/reproducing layer and received by a light receiving unit via an objective lens of the pickup device are combined together to form the RF signal. The RF signal is transmitted through a low-pass filter (LPF) 100 and is output as an average V1 with respect to a predetermined interval. Generally, during a recording or reproduction, the larger an amplitude of the average V1, the better it is. This is because the average V1 carries information recorded in an optical disc, and it is easier to read the information as the amplitude of the average V1 increases. Accordingly, a processor 200 monitors the amplitude of the average V1 while variously changing a magnitude of an error compensation signal V2 that is to be added to a focus error signal, and obtains the error compensation signal V2 where the average V1 is at a maximum. The obtained error compensation signal V2 is added to the focus error signal and is then input into a focus controller 300. Through this procedure, a focus error can be compensated where a recording and/or reproducing apparatus is recording or reproducing data.
However, the RF signal is not sensitive to a variety of errors, such as a focus error and an error due to a change in a thickness. More specifically, in the case of an optical disc for recording in which information is rarely recorded, a variation of an RF signal is very slight. Accordingly, the RF signal is not a proper reference signal for an accurate control, particularly, where a high density and mass capacity of an optical disc require an accurate error compensation.